The invention relates to a method and a fiber line system for producing chemical pulp in such a way that shive-containing rejects from screening are minimized and reprocessed in a cost-effective and advantageous manner. The invention has a number of advantages over previously known methods of treating chemical pulp after cooking for minimizing the amount of reject material in the final pulp produced.
U.S. Pat. No. 4,220,498 (the disclosure of which is hereby incorporated by reference herein) discloses a number of different alternatives for the treatment of chemical pulp after cooking in order to reduce the amount of reject material. In some sequences the pulp is screened prior to oxygen delignification and the rejects are subjected to further screening, refining, or accessory oxygen delignification. In other sequences the pulp is screened after oxygen delignification, but again is subjected to refining and typically other treatment, such as accessory oxygen delignification.
U.S. Pat. No. 4,895,619 (the disclosure of which is hereby incorporated by reference herein) suggests eliminating problems associated with handling and reintroducing the screen rejects from a screening stage that are associated with the U.S. Pat. No 4,220,498 by simply oxygen delignifying the rejects fraction exteriorly of the fiber line, and then returning it to the delignified rejects fiber line prior to the oxygen delignification stage in the fiber line.
U.S. Pat. No. 4,595,455 discusses a brown stock treatment process where the digested brown stock is first coarse screened, and then washed and pressed to high consistency, i.e. to about 30% consistency. After pressing the stock is fluffed into an oxygen delignification tower where the stock is delignified. The stock discharged from the delignification tower is diluted to low consistency and screened. The screened accepts are introduced to further processing in the main process line and the rejects are turned back to the feed of the brown stock press.
A significant drawback in the above-described process is the treatment consistency and its effect on the shives. Firstly, it should be understood that at high consistency (i.e. a consistency above 25%) delignification is a difficult task. In actual mill-scale operations it has been found that there are problems with the delignification efficiency in the high consistency delignification of fibers. Therefore, it is not a surprise that shives will not be loosened into fiber form but will maintain their original shape and xe2x80x9cstrengthxe2x80x9d.It could even be said that the only thing that tends to break the shives in U.S. Pat. No. 4,595,455 is the screening and other mechanical working on the shives. The reason for this is that when the rejects received from the screening stage after delignification are returned directly to the brown stock press, the shives have very little time, not nearly enough time, to be impregnated by the alkaline liquor. Dilution and pressing in U.S. Pat. No. 4,595,455 are virtually successive operations since the shives do not have time to be impregnated by the liquor and are not treated efficiently at all in the delignification tower (only the surface of the shives is treated, i.e. delignificated). Also, at a consistency of 30% there is no alkaline liquid around the shives so that there cannot be any efficient mass transfer between the liquid and the shives.
While the prior art procedures as described above are reasonably effective in ensuring a minimum of reject material in the final pulp produced, they have a number of drawbacks associated therewith. When refining is used the pulp produced by the refiner is really chemi-mechanical pulp rather than chemical pulp, which can make the final pulp produced have different properties than are desired. Also the use of a refiner significantly increases the air content of the pulp which impairs the runnability of the entire process. In all cases with the prior art techniques as described above it is necessary to invest in additional capital equipment aside from the conventional fiber line, which equipment can be very expensive. For example accessory (outside of the fiber line) oxygen delignification equipment and/or refiners are highly capital intensive.
Compared to the prior art in general, as well as the specific prior art discussed above, the method and system according to the present invention have a number of advantages. In particular according to prior art typical screening procedures (where a screening stage is provided directly after the brown stock washer) the pulp foams significantly, and it is necessary to utilize a significant amount of anti-foaming agent in order to control the foaming problem. According to the invention a minimal amountxe2x80x94and perhaps zeroxe2x80x94anti-foaming agent is necessary. Also, in the practice of the invention, because of the relatively low consistency during delignification (e.g. about 6-18%), there is always free liquid between the shives and fibers so that impregnation and mass transfer take place substantially throughout the process, not just during a short dilution/screening stage.
In many prior art techniques the pulp has already cooled off either before or during the thickening. For example vacuum washers have been utilized as thickeners. According to the present invention the pulp may be fed xe2x80x9chotxe2x80x9d into the oxygen delignification stage or stages, improving the heat economy of the process.
In the prior art, reaction products and chemicals which pollute the environment have often been discharged along with the rejects. According to the present invention considerably less reaction products are discharged, and the amount of reject material is smaller, providing a more environmentally sound approach.
The invention is advantageous compared to prior art systems and methods even where the screening stages are disposed after oxygen delignification because the capital investments associated with the refiners and/or accessory oxygen delignification equipment (as described above with respect to U.S. Pat. Nos. 4,895,619 and 4,220,498) are unnecessary.
The invention is also advantageous in that according to the invention shives are softened and partly disintegrated during oxygen delignification, reducing the amount of reject material; and the screening stages may actually be considered part of the bleaching process, even employing a chelating stage, thus enabling efficient individual fiber treatment. In fact it is conceivable that utilizing the teachings of the invention it would be possible to omit entirely the screening stages by using particular oxygen delignification treatment, so that pulp does not contain any shives after oxygen delignification.
According to one aspect of the present invention a method of producing chemical pulp is provided comprising the following steps: (a) Cooking comminuted cellulosic fibrous material (e.g. wood chips) to produce brown stock. (b) Washing the brown stock to produce chemical pulp. (c) Oxygen delignifying the chemical pulp at a consistency of between about 6-18% (preferably about 8-15%, e.g. about 10-13%). And, (d) screening the pulp from step (c) to produce at least an accept fraction and a shive-containing reject fraction. Steps (a) through (d) are practiced in a main fiber line, and then after step (d) there are the steps of: (e) Further treating (e.g. bleaching) the accept fraction, and (f) directly transporting the shive-containing reject fraction back to the main fiber line before step (c). Step (e) is typically practiced by bleaching the chemical pulp, with peroxide or other non-chlorine bleaching chemicals, and where peroxide bleaching is used a chelating stage is also employed. Step (c) is practiced at medium consistency (6-18%) to ensure sufficient alkaline liquid in the pulp during delignification so that impregnation and mass transfer between the liquid and shives takes place throughout at least all of steps (c) and (d).
The method according to the invention also typically is practiced in such a way that step (b) is practiced using a brown stock washer, and there is the further step of coarse screening the pulp with a coarse screen and washing the coarse rejects from the coarse screen; and wherein step (c) includes mixing oxygen with the medium consistency pulp in a mixer; and wherein step (f) is practiced to directly transport the reject fraction from step (d) to just before the mixer, or between the brown stock washer and the mixer, or to just before the coarse screen, or to the coarse reject washer. Also, steps (c)-(f) are preferably practiced to allow the reject fraction to be impregnated by alkaline liquid for enhancing the delignification and separation of fibers of and from shives.
Step (c) is typically practiced utilizing first and second distinct oxygen delignification, or more than first and second delignification, stages. A mixer is provided before the first stage and between the stages, with oxygen and typically other materials (such as magnesium, such as in the form of MgSO4, and typically alkali and steam, and may be even a small amount of hydrogen peroxide) are added. There typically is no between stage washing. Also step (c) is typically practiced utilizing upflow vessels and at least one of the upflow vessels comprises a multiple feeding device. The reject fraction from step (d) is acted upon between steps (a) and (d) utilizing mechanical action without refining, so that weak bonds of shives and the like are broken and so that a minimum amount of rejects (or perhaps none at all) are separated in the screening stage (d). The mechanical action is based on the use of medium consistency mechanical devices like the MC(copyright) ( mixers or MC(copyright) pumps which are mainly used for mixing chemicals with pulp or for transferring pulp from one process step to another.
The term xe2x80x9cdirectly transportingxe2x80x9d as used in the present specification and claims with respect to conveyance of pulp from after an in-line screening stage to before an in-line oxygen delignification stage means that the pulp is substantially only conveyed from one place to the other, e.g. by pumping or pressure differential, without refining or accessory oxygen delignification.
The term xe2x80x9cmechanical action without refiningxe2x80x9d as used in the present specification and claims relating to physical activity to which the pulp is subjected means to subject the pulp to one or more mixers (including possibly fluidizing mixers) and mechanical inlet or discharge devices (such as scrapers or other rotating blades or paddles), so that weak bonds of shives and the like are broken, but without the intense mechanical energy provided by refining, which in essence actually results in the production of chemi-mechanical pulp.
Also according to the present invention typically there is the step of washing the pulp after oxygen delignification. This washing may take place either prior to or after screening. Also a chelating agent may be added to the washer especially where step (e) will be practiced to include peroxide bleaching.
According to another aspect of the present invention a method of producing chemical pulp is provided comprising the following steps: (a) Cooking comminuted cellulosic fibrous material to produce brown stock. (b) Washing the brown stock to produce chemical pulp. And, (c) oxygen delignifying the chemical pulp preferably at between about 6-18% consistency so that the shives are properly impregnated; and wherein oxygen delignification is practiced utilizing at least first and second distinct oxygen delignification stages each comprising an upflow vessel, and at least one of the vessels including a multiple feeding device. And, (d) during the practice of step (c), subjecting the pulp to mechanical action without refining so as to produce an oxygen delignified chemical pulp substantially devoid of shives so that downstream screening of the oxygen delignified pulp is unnecessary. The mechanical action without refining is typically practiced utilizing a mixer before the first oxygen delignification stage, a mixer before the first and second stages, a multiple feeding device for one of the stages, and various scrapers, inlets, discharge devices, medium consistency pumps, and other mechanical of the vessels. The shearing forces created by such feed, discharge, and mixing devices are weak compared to those of refiners. Rather than disintegrating the shives into fibers as is done by a refiner, and the mechanical devices practicing this aspect of the invention are successful because the bonds in the shives have been weakened to such an extent via oxygen delignification that the relatively weak mechanical action of these elements can break the shives into fibers, substantially eliminating the need for screening at all (except for a coarse screen, e.g. before the brown stock washing).
According to another aspect of the present invention a chemical pulp producing fiber line system is provided comprising: A fiber line comprising in sequence: a digester for cooking cellulosic fibrous material to produce brown stock; a first washer for washing the brown stock from the digester; at least one oxygen delignification stage; and a screening stage for screening chemical pulp from the oxygen delignification stage to produce an accepts fraction and a shive-containing rejects fraction. And, means for directly transporting the shive-containing rejects fraction to the fiber line before an oxygen delignification stage, and after the brown stock washer.
In the system the at least one oxygen delignification stage preferably comprises first and second upflow oxygen delignification vessels, at least one of the vessels comprising a multiple feeding device; and means for subjecting the pulp to mechanical action without refining while oxygen delignifying the pulp.
In the system the digester comprises a single continuous digester or a plurality of batch digesters; and there is a coarse screen between the digester and the screening stage, and an oxygen delignified pulp washer between the at least one oxygen delignification stage and the screening stage, or just after the screening stage.
It is the primary object of the present invention to provide a simple, advantageous, and cost effective method and system for producing chemical pulp having a minimum of reject material in the final pulp produced. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.